The Auditory System of an Atympanate Bushcricket Phasmodes Ranatriformes (Westwood) (Tettigoniidae: Orthoptera)

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Abstract
The auditory system of the atympanate tettigoniid Phasmodes ranatriformes has been investigated using anatomical and physiological methods. The prothoracic tracheae show no specialization in terms of an acoustic spiracle, acoustic bullae or an enlargement of the trachea in the proximal tibia. Instead of a true tympanum, a depression with small cuticular protuberances is found on the proximal tibia of the foreleg. The complex tibial organ is formed from the three sensory organs usually found in tettigoniids. The crista acustica consists of about 16–18 receptor cells. Extracellular recordings from the leg nerve reveal sensitive responses to vibrational stimuli (at a threshold comparable to that found in other tettigoniids) only, and not to airborne sounds. The two possibilities either that this animal shows a secondary reduction of the auditory system or that it represents a primitive form are discussed. Developmental data from other tettigoniids seem to confirm the latter hypothesis.